1. Field of the Invention
The present invention relates to a method of resin molding and a resin molding machine for the method.
2. Description of Related Art
The applicant of the present invention has invented molding machines, in which parting faces of molding dies are covered with release film while molding (see Japanese Patent Kokai Gazettes No. 8-142105, No. 8-142109 and No. 9-57785). In the conventional resin molding machines, the parting faces, which include cavities, of the molding dies are covered with the release film, then resin molding is executed in the cavities. By using the release film, resin can be solidified without sticking to the molding dies. Further, molded products can be easily ejected from the molding dies, so no ejector pins are required and structures of the molding dies can be simpler. Many types of resin, e.g., tablet, pellet, liquid, can be employed in the conventional molding machines.
FIG. 31 is a front view of a conventional resin molding machine, in which no release film is used; FIG. 32 is a plan view thereof. In the resin molding machine, a part "A" is a press section for clamping and molding work pieces, e.g., lead frames, with resin; a part "B" is a work feeding section, in which the work pieces are fed; a part "C" is a tablet feeding section, in which resin tablets are fed; a part "D" is a loading section, in which the work pieces and the resin tablets are arranged and set into a molding die; a part "E" is an unloading section, in which molded products are taken out from the molding dies and disused resin of the products are removed; and a part "F" is an accommodating section for accommodating the molded products.
As described above, the resin molding machine using the release film has some advantages. For example, the molded products can be easily ejected from the molding die, and the structures of the molding dies can be simpler. The structure of the resin molding machine using the release film is quite different from that of the resin molding machine using no release film. To execute the method of resin molding with the release film, the old molding machines must be fully replaced, so the equipment cost must be quite higher.
However, quality of the molded products, which are molded by the resin molding machine shown in FIGS. 31 and 32, is not fully satisfied. In the case of molding lead frames with resin, the resin molding machine has the following disadvantages: resin flash is apt to be formed on the surface of the molded product; thickness of the molded products are not fixed; and it is difficult to mold thinner products.
In FIGS. 33-35, a work piece (a lead frame of transistors) 200 having heat sinks 202 are molded with resin. The work piece is clamped by molding dies and molded by filling the resin in cavities 206 via gates 204. Dam blocks 208 fill spaces between adjacent workpieces 200 so as not to leak the resin. The height of the dam blocks 208 is slightly lower than the thickness of the work piece 200 so as not to damage the opposite parting face of the molding die when the molding dies clamp the work piece 200. Thus, thin resin flash is formed between the dam blocks 208 and the opposite parting face.
The resin flash 210 is formed between the dam block 208 and the work piece 200 and on outer faces of the heat sinks 202. The resin flash 210 is also formed in the peripheral of resin paths connecting pots to the cavities 206, e.g., the gates 204, and in the peripheral of culls 212. This is because securely clamping the work piece 200 has priority over preventing the resin flash, so the resin flash 210 is apt to be formed between the molding dies.
In FIG. 36, a work piece (a plastic circuit board of BGA) 200 is molded with resin. The resin flash 210 is formed in the vicinity of molded parts 214. The thickness of the plastic circuit boards is not fixed, so all plastic circuit boards cannot be clamped with fixed clamping force. Therefore, resin flash maybe formed on surfaces of the molded products.
If the molded products have resin flash formed thereon, the step of removing the resin flash is required after a molding step. Furthermore, the resin flash is scattered in all directions when the molded products are taken out from the molding die; the scattered resin causes troubles. Further, if the resin flash is formed on a surfaces of an electric terminal, the molded product cannot be electrically connected.